Osteoclasts are terminally differentiated, multinucleated cells of monocytic origin. In this study, we report that osteoclasts secrete a 35 kD protein and that phorbol myristate acetate treatment stimulates secretion dramatically. Peptide digests of the protein were analyzed by mass spectroscopy. The protein was identified as myb induced myeloid protein-1 precursor (mim-1 protein). Mim-1 is expressed specifically by hematopoietic cells and has no known function. It is homologous with the neutrophil chemokine, chondromodulin II, which stimulates proliferation of osteoblasts and chondrocytes. Western analysis showed that osteoclasts secrete mim-1 into culture media. Immunofluorescence studies demonstrated an cytoplasmic and perinuclear distribution of mim-1 in both avian osteoclasts and human osteoclast-like cells. Expression and secretion of a chemokine-like protein by osteoclasts suggests a novel signaling pathway in the bone microenvironment that may be involved in coordinating bone remodeling.